Siniy
The following tiddler seems to be where the new field creation logic is. If manipulated you may be able to create other fields but it is not clear what implications this would have. Test on an empty.html and save and reload, list and search then ask the dev team if they can tell you what the consequences are. It is possibly aligned with the rules that apply to Javascript and html. How do they handle alternate languages?
A workaround may be to use indexes in a data tiddler to define fields with other index values such as a fieldname, then the value stored will be equivalent to a named field, I do not expect the same restrictions to apply.
However a quick test like this {{{ [[data]getindex[цвет]] }}} did work on a tiddler named data containing цвет: красный with the type application/x-tiddler-dictionary.
The Query did return красный
tiddlername
$:/tiddlername/datafields (a data or JSON tiddler
Use index as fieldname, value as fieldvalue
Use serial number and/or relink plugin with additional settings, if tiddlername rename is needed
There is a fair argument that fieldnames could use alternate language alphanumeric's but the code refactoring may be prohibitive and this may be for future major versions of tiddlywiki.
Regards
Tones